SAW filter using tetrahedral amorphous carbon and method for manufacturing the same

ABSTRACT

Disclosed is an element using a piezoelectric characteristic, and in particular, an SAW filter and a method for manufacturing the same. The SAW filter according to the invention is resistant to input wave of high power by employing ta-C or CNT as an acoustic wave transmission medium. The method for manufacturing the SAW filter according to the invention simplified the manufacturing process and reduced a transmission loss as well noise.

This Application is a Divisional of application Ser. No. 09/942,637,filed on Aug. 31, 2001, now U.S. Pat. No. 6,566,983.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface acoustic wave (SAW) filter,and in particular, to an SAW filter for high frequency that realized anacute skirt characteristic and resistance to input wave of high power byemploying a carbon nanotube (CNT) as an acoustic wave transmitting mediaof the SAW filter as well as by enlarging the space between electrodesof inter-digital transducers.

The present invention also relates to a method for manufacturing an SAWfilter that simplifies a manufacturing process by omitting a surfaceabrasive process, shortens a manufacturing process, and lowers amanufacturing cost due to the short manufacturing process by using atetrahedral amorphous carbon (ta-C) as a transmitting medium of the SAW,while facilitating formation of a wide area and reducing transmissionloss and noise.

2. Description of the Prior Art

An SAW filter is an element used for removing noise and interferencethrough selective pass of the bandwidth of a particular frequency(center frequency) by using piezoelectric characteristics that areinter-transducible between electricity and machinery. SAW filters areused for core parts of TV, VCR, mobile telecommunications, etc. asbandpass filters.

FIG. 1 is a schematic diagram of an SAW filter in general.

As shown in FIG. 1, the SAW is an element for transducing electricsignals to acoustic wave along the surface of a piezoelectric substrate103 by taking an advantage of a total distortion effect (an inversepiezoelectric effect) of the substrate through installation of twointer-digital transducers IDT 101, 102 on the piezoelectric substrate,and detecting the generated SAW as an electric signal by means of anoutput transducer.

Here, the IDT comprises a transmitting IDT 101 and a receiving IDT 102.The transmitting IDT 101 transduces the electric signals inputted to asignal generator to an SAW. The transduced SAW progresses along thesurface of the piezoelectric substrate 103 so as to be transmitted tothe receiving IDT 102.

As a consequence, the receiving IDT 102 transduces the SAW signals toelectric signals again by using the piezoelectric effect. Here, thereceiving IDT 102 performs a wave filtering so that frequencycharacteristics can be determined by a geometrical structure of the IDTelectrode.

FIG. 2 is a graph illustrating frequency pass characteristics of the SAWfilter in general.

As shown in FIG. 2, the SAW filter is set to pass transmitted signalsonly of a particular frequency, and used as a bandpass filter with setamplitude and phase.

Also, the materials most widely used for a substrate of the SAW filterare LiTaO3 (LTO) and LiNbO3 (LNO), which are mono-crystal materials.However, the SAW filter using such mono-crystal materials cannot be usedfor high frequency greater than GHz. Therefore, new materials are beingdeveloped in a large scale.

Meanwhile, the center frequency of the SAW is proportional to a velocityof the SAW in a transmission medium but inverse proportional to thespacing between the IDT electrodes. Therefore, it is necessary to eitheruse a material of high elasticity or narrow the spacing between the IDTelectrodes to heighten the frequency band of the center frequency.

However, there exists a limitation to reduce the spacing between IDTsdue to the limitation of patterning technology of IDT electrodesderiving from the limitation of lithography technique as well as to theproblems in securing stability and durability against high pressurepower. For these reasons, diverse researches are being conducted in amanner of introducing a medium of high elasticity.

Also, in the basic SAW filter, piezoelectric material was a transmittingmedium that takes charge of piezoelectric effect and total distortioneffect (inverse piezoelectric effect). However, no piezoelectricmaterial of high elasticity has been developed to date in light of themechanical elastic characteristics of the piezoelectric material.Therefore, new media are being introduced to transmit the SAW.

Outstanding examples of that case are a “piezoelectric material (mainlyZnO)/diamond film” structure, a “piezoelectric material (mainlyZnO)/sapphire” structure, and an “LiNbO3/diamond” structure that haveintroduced either diamond or sapphire as an SAW medium.

With this introduction, a notably higher transmission velocity(5,200-5,700 m/sec in case of sapphire, and 9,000-11,900 m/sec in caseof diamond) than the existing Quartz (3,158 m/sec), LNO (3,488 m/sec)etc. could be achieved. Thus, filters became available in higherfrequency band.

While employing diamond for a medium of the SAW has an advantage ofavailing the SAW filter in high frequency band, however, it also posesthe following problems.

First, the process of synthesizing diamond requires high temperature.Also, because of a deflection of the substrate caused by stress of thediamond, it is difficult to realize an area to be greater than 4 inches,for instance.

Further, since the diamond is poly-crystal, a grain boundary exists,thereby increasing the electric wave loss of the signal. Also, despitethe need to undergo abrasion of the rough surface of the diamond for useas a medium of the SAW, it is difficult to perform the abrasion processdue to hardness of the diamond as well as to consumption of time andexpense.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide an SAWfilter for high frequency that realized an acute skirt characteristicand resistance to input wave of high power as well as to high frequencyhigher than 2 GHz by employing a carbon nanotube as an acoustic wavetransmitting media of the SAW filter by enlarging the space betweenelectrodes of inter-digital transducers.

To achieve the above object, there is provided an SAW filter accordingto one aspect of the present invention, comprising: a substrate; acomplex film composed of CNT and a piezoelectric material, and having apiezoelectric characteristic so as to transmit SAW; and IDTs foroutputting electric signals by receiving the SAW from the complex film.

There is also provided a method for manufacturing an SAW filter,comprising the steps of: forming a complex film on a substrate; forminga conductive material on the complex film; and patterning the conductivematerial.

Here, the complex film is formed either of CNT and a piezoelectricmaterial or of a complex comprised of CNT and an insulator and apiezoelectric material formed on the complex.

The method for forming the complex film according to one aspect of thepresent invention comprises the steps of: forming catalyst patterns bymeans of a lithography technique used in an ordinary semiconductorprocess; growing a CNT bridge in horizontal direction between thecatalyst patterns; and forming a piezoelectric material on the grown CNTbridge by sputtering.

The method for forming the complex film according to another aspect ofthe present invention comprises the steps of: forming suspension bydispersing the CNT into a predetermined solution; attaching the CNTsuspension to the substrate by means of electromagnetic field orelectrostatic force; and forming a piezoelectric material on thesubstrate, to which the CNT suspension has been attached, by sputtering.

The method for forming the complex film according to another aspect ofthe present invention comprises the steps of: forming suspension bydispersing the CNT into a predetermined solution; precipitating the CNTsuspension by filtering through a filter membrane; and forming apiezoelectric material on the substrate, in which the CNT has beenprecipitated, by sputtering.

In the process of filtering the CNT suspension through the filtermembrane, the CNT suspension is aligned by means of magnetic field.

There is also provided a SAW filter according to another aspect of thepresent invention, comprising: a substrate; an acoustic wavetransmission medium of SAW composed of tetrahedral amorphous carbon andformed on the substrate so as to transmit SAW; a piezoelectric materialhaving piezoelectric characteristics and formed tightly onto the SAW;and IDTs for generating SAW by transmitting inputted electric signals tothe piezoelectric material, and outputting the electric signals byreceiving the SAW from the piezoelectric material.

Here, the acoustic wave transmission medium of the tetrahedral amorphouscarbon formed on the substrate is deposited by arc discharging withrespect to black lead. The acoustic wave transmission medium of thetetrahedral amorphous carbon formed on the substrate has a thicknessless than 1 μm.

The acoustic wave transmission medium of the tetrahedral amorphouscarbon formed on the substrate is deposited by laser ablation usingblack lead.

Thus, the present invention described as above can provide an SAW filterfor high frequency that has an acute skirt characteristic and isresistant to an input wave of high power by employing a CNT as anacoustic wave transmission medium of the SAW filter and by enlarging thespace between electrodes of inter-digital transducers.

The present invention relates to an SAW filter employing a CNT (havingan elasticity of 1.8 Tpa), which is known to be a material of highestelasticity as an acoustic material of the SAW filter. The CNT can beproduced to have a variety of elasticity depending on the structure anddiameter thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a schematic diagram of an SAW filter in general;

FIG. 2 is a graph illustrating frequency pass characteristics of the SAWfilter in general;

FIG. 3 is a schematic diagram illustrating a process of manufacturing anSAW filter by using a CNT according to the present invention;

FIG. 4 is a schematic diagram exemplifying a variety of structures of anSAW filter using a CNT according to the present invention;

FIG. 5 is a diagram illustrating a process of horizontally growing a CNTbetween catalyst patterns to manufacture an SAW filter by using the CNT;

FIG. 6 is a diagram illustrating a process of manufacturing a buckypaper of the CNT aligned in an axial direction by using magnetic fieldto manufacture an SAW filter using the CNT according to the presentinvention;

FIG. 7 is a diagram illustrating a process of manufacturing an SAWfilter by using tetrahedral amorphous carbon according to the presentinvention;

FIG. 8 is a schematic diagram exemplifying a variety of structures of anSAW filter using tetrahedral amorphous carbon according to the presentinvention; and

FIG. 9 is a schematic view of a deposition apparatus for depositing thetetrahedral amorphous carbon in manufacturing an SAW using thetetrahedral amorphous carbon according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described hereinbelow with reference to the accompanying drawings. In the followingdescription, well-known functions or constructions are not described indetail since they would obscure the invention in unnecessary detail.

FIG. 3 is a schematic diagram illustrating a process of manufacturing anSAW filter using a CNT according to the present invention.

Referring to FIG. 3, the SAW filter using a CNT according to the presentinvention forms a film composed of “a complex of a CNT and apiezoelectric material” on a silicon substrate at a thickness ranged0.1-1 μm, for example, as shown in FIG. 3(A).

As shown in FIG. 3(B), Al is formed on the complex film by means ofevaporation or sputtering to form IDT electrodes. Also, IDT electrodeshaving an appropriate shape are formed by means of lithography as shownin FIG. 3(C).

If necessary, silicon oxide or silicon nitride is provided for theelement formed as shown in FIG. 3(C). Here, silicon oxide or siliconnitride serves to heighten or lower the central frequency as well as toenhance temperature resistance of the element. Further, the siliconoxide or silicon nitride has advantages of electric insulation and easytransportation of the element.

Meanwhile, the structure of the SAW filter according to a preferredembodiment of the present invention is variable into diverse shapes asshown in FIG. 4. FIG. 4 is a schematic diagram exemplifying a variety ofstructures of an SAW filter using a CNT according to the presentinvention.

According to a varied embodiment of the present invention, the IDTelectrodes are shielded, if necessary, and a piezoelectric material suchas ZnO is formed as a separate layer. Also, it is obvious that employingthe CNT as an acoustic material is applicable to the SAW filter of manyknown structures.

Meanwhile, in order to manufacture an SAW filter by using the CNT as anacoustic material, it is important to transduce the CNT to be used as anacoustic material to a sequential elastic medium. Here, a complex of theCNT and the piezoelectric material or a complex of the CNT and aninsulator may be used for the SAW filter. The following is a descriptionof the methods for manufacturing a CNT having a sequential elasticmedium.

The first method is to manufacture a CNT bridge by means of in-situ incatalyst patterns produced by an ordinary semiconductor process.

This method uses a lithography technique commonly used in the siliconsemiconductor process to form catalyst patterns as shown in FIG. 5. ACNT bridge is grown in horizontal direction between the catalystpatterns. FIG. 5 is a diagram illustrating a process of horizontallygrowing a CNT between catalyst patterns to manufacture an SAW filter byusing the CNT.

The CNT is grown in a row in horizontal direction, as shown in FIG.5(B), between the pre-manufactured catalyst patterns, as shown in FIG.5(A). A piezoelectric material such as ZnO or PZT is formed thereon bysputtering so as to be used for an SAW filter.

Another available method in addition to the method for forming a filmcomposed of a complex of the CNT and the piezoelectric material is tofirst form a complex of the CNT and the insulator and to form apiezoelectric material thereon.

When forming a complex of the piezoelectric material, a mechanicalconnection between the CNT and the piezoelectric material is veryimportant. If the mechanical connection between the CNT and thepiezoelectric material is unstable, the acoustic wave is not effectivelytransmitted from the piezoelectric material.

The complex of the CNT and the piezoelectric material manufacturedthrough the above process is formed as shown in FIG. 3(D). In that case,an Al electrode needs to be formed on the complex. Thus, the surface ofthe complex, on which the electrode is to be formed, should maintainelectric insulation, and it can be realized by sputtering thepiezoelectric material to have a thickness sufficient for electricinsulation.

The second method is to use the CNT powder.

Here, this method will be referred to as “a CNT powder suspensionmethod” for convenience sake. According to the method, the CNT powderdispersed in the solution is adsorbed by electric force comprisingelectrostatic force (Coulomb force) or electric field. This method is amethod of attaching a suspension formed by mixing the CNT powder, whichhas been produced by an electric arc discharge, a laser evaporation orCVD, etc., with sodium dodecyl sulfate (SDS) or lithium dodecyl sulfate(LDS), and by dispersing the same by using ‘an electrophoresis’, whichuses electric force, or ‘a self-assembly’, which uses an electrostaticforce with a chemical functional group, to a desired location.

The preferred embodiment of the present invention uses an LDS solutionor an SDS solution of 1 weight % as an aqueous solution. The solutionwas oscillated for two minutes by an ultrasonic oscillating device. If asilicon wafer is dipped into the solution and taken out thereof, a CNTis attached to the silicon substrate.

In case of a self-assembly, the silicon substrate is dipped into asolution of 2.5 mM/liter 3-aminopropyltriethoxysilane (APS) for 15minutes under a room temperature. This is a process of producing anamino group, which has been charged with positive ions on the substrate.

The amino group charged with positive ions generates an electrostaticforce with a sulfate group charged with negative ions, which is adsorbedonto the CNT, so that the CNT can be attached to the substrate. Here,the amount of CNT attached to the substrate is proportional toconcentration and time. A sulfactant attached to the substrate can bedetached therefrom by lightly rinsing with pure water and drying.

In case of the electrophoresis, desired metal patterns are formed on theSiO₂ substrate, and the CNT charged with negative ions by the electricforce due to a positive voltage is drawn and attached to the substrate.

A piezoelectric/elastic material structure can be manufactured bysputtering a piezoelectric material on the substrate, to which the CNThas been attached. An SAW filter can be manufactured by forming Al IDTsthereon.

If necessary, insulation film such as silicon oxide or silicon nitridecan further be formed as shown in FIG. 4. The effects of the siliconoxide lie in lowering the center frequency and securing stabilityagainst temperature, while those of the silicon nitride lie inheightening the center frequency and resistance against humidity.

The third method is a method of manufacturing a bucky paper well alignedin one direction by aligning the CNT powder with external magneticfield.

The basic manufacturing principle is to precipitate the CNT suspensionof a liquid state into a nylon filter membrane. If satisfactory, avariety of filters may be used instead of nylon.

A method use for manufacturing a bucky paper through precipitation is toforcibly stream the suspension to heighten the manufacturing velocity.For example, if a tube of a cylindrical shape is prepared as shown inFIG. 6, and a filter membrane is installed perpendicular to a tube axiswhile streaming the water flow in the direction parallel to the tubeaxis, the bucky paper can be manufactured faster than the flow velocity.

FIG. 6 is a diagram illustrating a process of manufacturing a buckypaper of the CNT aligned in an axial direction by using magnetic fieldto manufacture an SAW filter using the CNT according to the presentinvention.

Here, the flow direction may be variable to be in tube axial directionor radial direction. It is possible to obtain a bucky paper well alignedin one direction by bridging strong magnetic field from outside.

For instance, the CNT is aligned in the magnetic field direction due tothe catalyst metal of strong magnetic force included in the CNT ifstrong magnetic field of 25 tesla is bridged from outside to beperpendicular to the tube axis and the above process is performed.

As described above, the SAW filter using a CNT according to the presentinvention realized an SAW filter that is useful for high frequency, hasan acute skirt characteristic, and is resistant to input wave of highpower by employing a carbon nanotube (CNT) as an acoustic wavetransmitting media of the SAW filter and by enlarging the space betweenelectrodes of inter-digital transducers.

Further, the SAW filter using a CNT can ignore bulk acoustic waveprogressing into a substrate because of a slim thickness thereof. Thus,the SAW filter using a CNT has advantageous effects of reducing loss andnoise caused by bulk acoustic wave.

FIG. 7 is a diagram illustrating a process of manufacturing an SAWfilter by using tetrahedral amorphous carbon according to the presentinvention.

Referring to FIG. 7, the SAW using tetrahedral amorphous carbon (ta-C)according to the present invention is manufactured by the followingprocess. First, the ta-C is deposited on a silicon substrate at athickness ranged 0.1-1 μm, for instance, by means of direct current arcdischarging, as shown in FIG. 7(A).

As a next step, as shown in FIG. 7(B), Al is formed on a ta-C film bymeans of evaporation or sputtering to form IDT electrodes. Also, IDTelectrodes of an appropriate shape are formed by means of lithography asshown in FIG. 7(C).

As shown in FIG. 7(D), a ZnO film, which is a piezoelectric material, isformed to have a thickness ranged 0.3-2.5 μm, for instance. A SAW filterusing the ta-C as an SAW transmission medium can be manufactured in thismanner.

Meanwhile, the structure of the SAW filter as described with referenceto an embodiment of the present invention is variable into diversestructures as shown in FIG. 8. FIG. 8 is a schematic diagramexemplifying a variety of structures of an SAW filter using tetrahedralamorphous carbon according to the present invention.

According to a modified embodiment of the present invention, the IDTelectrodes are shielded or a surface of the ZnO film undergoes hardeningabrasion. Also, it is obvious that employing the ta-C as an acousticwave transmission medium is applicable to the SAW filter of variousstructures that have been well known, though not illustrated in FIG. 8.

The ta-C employed as an acoustic wave transmission medium according tothe present invention is a kind of diamond-like carbon (DLC), but is anovel material having a 85% elasticity of a mono-crystal diamond. Such ahigh elasticity is attributable to the fact that more than 85% ofcarbonic composition within the material is comprised of SP3composition.

FIG. 9 is a schematic view of a deposition apparatus for depositing thetetrahedral amorphous carbon in manufacturing an SAW using thetetrahedral amorphous carbon according to the present invention. Thefollowing is a brief description of a process of depositing the ta-C onthe substrate made with reference to FIG. 9.

A black lead target was used as a carbon source in the embodiment of thepresent invention. Also used were substrate bias direct current and HF,RF, etc. for arc discharge. In case of the direct current, a voltage of−100V was applied to adjust a kinetic energy of carbon ions to be ranged100-125 eV. In case of HF or RF, however, a self-bias value wascontrolled to be within the same range.

Also, the substrate was cleaned by using Ar ion for 30 seconds to 5minutes. The substrate was water-cooled and rotated for uniform coating.For uniform coating of the area, arc beam raster was performed inhorizontal and vertical directions. The raster frequency was set to be50-60 Hz in horizontal direction and 2-16 Hz in vertical direction.

Also, the degree of vacuum before arc deposition was to be 10−7 torr,while the degree of vacuum during arc deposition was to be 10−3 or 10−4torr.

Another available method in addition to the method using arc dischargeis a deposition using laser ablation. However, the method using arcdischarge is much simpler in manufacturing the SAW filter with respectto area.

On the other hand, the SAW filter using ta-C as an SAW medium has thefollowing advantages.

First, the process of manufacturing an SAW filter by using the ta-C doesnot require so high temperature as in the diamond synthesizing process.This process also has a merit of undergoing a simple process notrequiring so long period of time as the mono-crystal growing processsuch as LNO or LTO.

Further, in the diamond synthesizing process, it is difficult to realizea wide area wider than 4 inches due to a bending of the substrate causedby a stress. In case of a mono-crystal, actual production is made in 3inches, although researches have been completed for 4 inches. However,using the ta-C coating enables manufacture of mono-crystals up to 9inches. Further, laying out of Arc enables uniform coating up to 30cm*30 cm. It is also possible to manufacture the mono-crystals widerthan 30 cm*30 cm.

Also, attachment of a sputtering device to vacuum arc depositionequipment enables sputtering of a piezoelectric material or an Alelectrode, thereby serving to simplify the process. A diamond, which isa poly-crystal, requires a post-abrasion. On the other hand, ta-Ccoating results in a very smooth surface composed of atoms having a aroot mean square (RMS) surface luminance of about 5 Å, thereby requiringno post-abrasion process.

Also, ta-C is chemically very stable and highly adhesive with thesubstrate.

Meanwhile, most of the SAW filters have a resistance up to 3.5 W due toa spacing between IDT electrodes. The preferred embodiment of thepresent invention used a material of high elasticity, and a transmissionvelocity of SAW is fairly high. Therefore, when the desired centerfrequency is the same, the spacing between IDT electrodes can bewidened. For instance, assuming that other conditions are the same, theIDT electrodes may be spaced twice wider than mono-crystals of LNO orLTO. Those electrodes are resistant to higher input power.

Therefore, the present invention has an advantage of lowering themanufacturing cost due to the process that is simple and requirestemperature lower than 200° C.

The ta-C SAW filter manufactured by the above method has a centerfrequency of about 2.3 GHz when the wavelength is 4 μm, i.e., when thespacing between the IDT electrodes is 2 μm and the IDT finger width is0.9 μm. The insertion loss was low to be about 10 dB, while the SAWtransmission velocity was ranged 8,500-9,000 m/sec. The electro-mechaniccombination coefficient was about 1.4%, while the Q value was about 600.The frequency deviation was merely about 1,000 ppm within thetemperature ranged −40° C.-85° C.

As described above, the SAW filter using tetrahedral amorphous carbonaccording to the present invention has advantageous effects ofsimplifying the manufacturing process dispensing with a surface abrasionprocess by employing tetrahedral amorphous carbon as an SAW transmissionmedium as well as of lowering the manufacturing cost and facilitatingformation of a wide area while reducing the transmission loss and noise.

While the invention has been shown and described with reference tocertain preferred embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A method for manufacturing a surface acousticwave (SAW) filter, comprising: forming a tetrahedral amorphous carbon(ta-C) film on a substrate, wherein the ta-C film is deposited on thesubstrate by means of arc discharge with respect to black lead; forminga conductive film on the ta-C film; patterning the conductive film; andforming a piezoelectric material on the patterned conductive film. 2.The method of claim 1, wherein the ta-C film is formed to have athickness less than 1 μm.
 3. The method of claim 1, wherein thepiezoelectric material is composed of ZnO to have a thickness less than2.5 μm.
 4. The method of claim 1, wherein the surface of thepiezoelectric material undergoes a hardening abrasion.
 5. A method formanufacturing a surface acoustic wave (SAW) filter, comprising: forminga tetrahedral amorphous carbon (ta-C) film on a substrate, wherein theta-C film is formed to have a thickness less than 1 μm; forming aconductive film on the ta-C film; patterning the conductive film; andforming a piezoelectric material on the patterned conductive film. 6.The method of claim 5, wherein the piezoelectric material is composed ofZnO to have a thickness less than 2.5 μm.
 7. The method of claim 5,wherein the surface of the piezoelectric material undergoes a hardeningabrasion.
 8. A method for manufacturing a surface acoustic wave (SAW)filter, comprising: forming a tetrahedral amorphous carbon (ta-C) filmon a substrate, wherein the ta-C is deposited on the substrate by meansof laser ablation using black lead; forming a conductive film on theta-C film; patterning the conductive film; and forming a piezoelectricmaterial on the patterned conductive film.
 9. The method of claim 8,wherein the piezoelectric material is composed of ZnO to have athickness less than 2.5 μm.
 10. The method of claim 8, wherein thesurface of the piezoelectric material undergoes a hardening abrasion.11. A method for manufacturing a surface acoustic wave (SAW) filter,comprising: forming a tetrahedral amorphous carbon (ta-C) film on asubstrate; forming a conductive film on the ta-C film; patterning theconductive film; and forming a piezoelectric material on the patternedconductive film, wherein the piezoelectric material is composed of ZnOto have a thickness less than 2.5 μm.
 12. The method of claim 11,wherein the surface of the piezoelectric material undergoes a hardeningabrasion.
 13. A method for manufacturing a surface acoustic wave (SAW)filter, comprising: forming a tetrahedral amorphous carbon (ta-C) filmon a substrate; forming a conductive film on the ta-C film; patterningthe conductive film; and forming a piezoelectric material on thepatterned conductive film, wherein the surface of the piezoelectricmaterial undergoes a hardening abrasion.